Most large caliber guns have a barrel with a rifled internal bore that imparts a stabilizing spin on an expelled projectile. The internal bore may be coated with a hard facing material, such as chromium, to minimize erosive wear increasing the number of projectiles that may be fired from the gun.
The typical large caliber projectile has a diameter slightly less than the diameter of the internal bore. One or more obturator, or rotating, bands gird the circumference of the projectile. At the bands, the diameter of the projectile is slightly larger than the internal diameter of the gun barrel. When the projectile is expelled, the rotating band is engraved by the rifling, contacting the rifling throughout the length of the tube imparting the projectile with a stabilizing spin.
The gun barrel is manufactured from a material such as steel and sometimes coated with a hard material such as a chromium facing. The gun barrel is harder than the rotating band which is typically copper or a copper alloy. As a result, a portion of the copper from the rotating band is deposited on the rifling inside the gun barrel. This copper deposition referred to as "copper fouling" can affect the ballistics of the projectile and major fouling can prevent the projectile from being inserted and seated, positioned in the barrel prior to firing, properly.
Copper fouling is currently a major problem for large artillery weapons, such as 155 millimeter howitzers, and is also noted in small and medium caliber cannons, such as 20 millimeter canons. The current solution to copper fouling is including a decoppering agent in the propellant charge. The decoppering agent removes the copper without damaging the gun barrel or the rifling.
A common decoppering agent is a sheet of lead foil deposited between the propellant and the projectile. On ignition of the propellant charge, the lead is vaporized and diffuses into the copper. The resultant alloy is brittle and easily shattered. The combination of the heat generated by the burning propellant and the mechanical movement of the propellant gases separates the brittle lead/copper alloy from the surface of the barrel. The fractured debris is swept from the muzzle of the gun with the propellant gases.
A second theory as to why lead foil is effective as a decoppering agent is that the heat generated by the burning propellant melts the lead foil. Liquid lead contacts the copper deposition and dissolves the copper, the copper bearing lead solution is expelled as a liquid from the muzzle with the propellant gases.
While metallic lead and lead compounds are effective decoppering agents, the materials are toxic to humans working around the weapons. There is a need for a lead free decoppering agent.
Among the lead free decoppering agent that have been proposed are bismuth, bismuth subcarbonate (BiCO.sub.3), tin and tin alloys. Bismuth compounds are very brittle and even metallic bismuth cannot be rolled into a thin foil like lead. Alloys of bismuth metal with other metals can be rolled into a foil, but the alloys are very expensive and less effective as a decoppering agent.
There remains, therefore, a need for a method to effectively introduce a lead free decoppering agent into a propellant charge and provide this decoppering agent with a flexibility and a desired shape not achievable with the prior art lead free decoppering agents.